What can cause focussing problems as children grow? As adults age?

Explain why a person who is near- sighted can see a nearby object clearly, but not a distant one?

A person who is near-sighted can see a close object clearly, but not a distant one because the lens converge the rays in front of the retina. Before they strike the retina, the rays have spread out and have formed a blurry image.

Explain why a person who is far-sighted can see a distant object clearly, but not a close one?

A person who is far-sighted can see a distant object clearly, but not a close one because the eye has a shorter shape than a normal one so, the lens forms the image behind the retina, and the image becomes fuzzy.